1. Field of the Invention
The invention relates to a writing head for electrostatic point printers of the type in which the writing head is fixed and comprises at least one row of point electrodes in front of which a printing support (dielectric sheet or paper having a dielectric layer) passes.
In certain electrostatic printers of this type counter-electrodes co-operate with the electrodes to cause electrostatic discharges which sensitize the printing support the counter-electrodes are disposed opposite the printing head electrodes on the other side of the printing support. This printing support therefore passes between the head and the counter-electrodes and remains in contact with the latter, but is generally separated from the electrodes by a gap in the range of several microns. An electrical voltage is always provided between all the electrodes and counter-electrodes; however the support opposite a given electrode is only sensitized when this voltage exceeds a predetermined threshold value.
In the case of a second type of electrostatic printers of the above-mentioned type, the counter-electrodes are strips supported by the printing head to form at least one row which passes along a row of electrodes. As the printing support moves, it remains in contact with both the electrodes and counter-electrodes.
In both categories of printers, a counter-electrode co-operates with a group of electrodes. In order to decrease the number of electronic switches enabling the electrode supply to be ensured, use is made of multiplexing possibilities. More precisely, an electrical control pulse for a particular electrode is directed toward this electrode by two switches, one of which controls a conductor network associated with a counter-electrode or with the corresponding group of electrodes. The other switch controls a circuit for an electrode of predetermined rank in each of these groups.
The writing heads of the printers belonging to the above-mentioned two categories type are prismatic and moulded. The electrodes are parallel points disposed in this prism. Operative ends of the electrodes are flush with a section or a ridge of the prism; the electrode assembly include at least one comb of enamelled conductor wires coated with a dielectric material such as epoxy resin. When the head supports counter-electrodes the counter-electrodes includes metallic tracks parallel to the row of corresponding electrodes. These moulded heads are of a complicated construction, as it is difficult to maintain the alignment, the parallelism and the equidistance of the electrode wires with the required accuracy before and during the moulding operation. In addition, multiplexing requires numerous interconnections between the wire. As the pitch of the electrodes decreases (i.e., as the axial distance between enighboring electrodes decreases) the connection points become more difficult to construct. It is noted that the connection points are the more difficult to construct, the smaller the pitch of the electrodes (i.e. their axial distance). In the case of modern printers the pitch of a row is approximately 0.25 mn, event though the electrodes are disposed in a staggered formation (i.e. in two parallel, staggered rows) to prevent the risk of inadvertent discharges between neighbouring electrodes.
2. Description of the Prior Art
It has already been proposed (see for example the article of Schayes and Gustin, ACTA ELECTRONICA, No. 21, January 1978, pages 93-96) to construct printed circuit printing heads. A printed circuit formed on a dielectric substrate, comprises two networks of narrow, parallel metallic tracks. These two networks, juxtaposed end to end on the substrate, have a common straight line boundary. The pitch of each of the networks is double the pitch determined for the electrodes; they are offset with respect to each other by an electrode pitch, i.e. half a network pitch. The substrate is then cut along the common boundary and the two printed half-circuits are pressed and glued, with the tracks facing inwardly, with the interposition of a thin insulating layer. A sandwich obtained in this way in which all the tracks are inserted, is then rectified to make the track ends constituting the electrodes flush. The electrodes are interconnected by means of metallic tracks printed on the external surfaces of the sandwich and connected to the electrodes by metallized plated walls on through holes provided through the substrate material.
The above-described method counters the difficulties associated with positioning of the wires and electrode interconnections which arise in the moulding method. However, the pressing and gluing step requires great precautions. It is, in effect, the necessary to monitor the position of the two half-circuits to very strict tolerances and to be sure that the pressing and gluing step does not cause the two half-circuits to slip with respect to each another. In addition, the possible positioning of counter-electrodes on the head in the vicinity of the electrodes may only be carried out after rectification. The above document does not mention such counter-electrodes. In effect, this method would appear to be difficult to apply to the construction of heads supporting counter-electrodes.